1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to the so-called air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) by a feedback control in accordance with the composition of the exhaust gas and operating conditions of the engine since an optimum operation of the three-way catalytic converter is enabled at the stoichiometric air-fuel ratio. The air intake side secondary air supply system is a system in which the air-fuel ratio is controlled by varying the amount of a secondary air through a secondary air passage connected to the carburetor on the downstream side of the throttle valve. As an example, Japanese Patent Application No. 57-200137 of the same applicant discloses a system in which an air control valve whose opening degree varies with the magnitude of a vacuum applied to the vacuum chamber thereof is provided in the air intake side secondary air supply passage communicated to the intake manifold on the downstream side of the throttle valve. In this system, an integral control of the air-fuel ratio is performed in such a manner that the air-fuel ratio is detected from the oxygen concentration of exhaust gas to produce an air-fuel ratio signal, and one of first and second control pressures respectively for gradually increasing the sectional area of the air intake side secondary air passage and for gradually decreasing the sectional area thereof, is supplied to the pressure chamber of the air control valve according to the content of the air-fuel ratio signal.
In this system, a constant vacuum or a control vacuum varying with the operating state of the engine is used as the first control pressure, and an atmospheric pressure is used as the second control pressure. The vacuum and the atmospheric pressure are switched by a three-way valve in accordance with the content of the air-fuel ratio signal and applied alternatively and intermittently to the vacuum chamber of the air control valve via a pressure passage. Thus, the air intake side secondary air is continuously supplied to the engine and the air-fuel ratio is controlled to the stoichiometric value by the feedback operation.
In addition, in the air intake side secondary air supply system, an open loop control in which the air-fuel ratio is controlled to the rich side is selected during a low load operation of the engine such as in a period of deceleration of the engine, because the combustion condition tends to be unstable in such a period. In the case of the open loop control, the atmospheric pressure which is from the three-way solenoid valve is continuously supplied to the pressure chamber of the air control valve via a pressure passage. Thus, the air control valve is closed and the supply of the secondary air is stopped.
In this operation, at a time when the vacuum is stopped and the supply of the atmospheric pressure to the pressure chamber of air control valve is started for the enrichment of the air-fuel ratio, the pressure level within the vacuum chamber gradually decreases to reach the atmospheric pressure due to the effect of a residual vacuum of the pressure chamber. Thus, there is some delay of the closure of the air control valve after the start of the application of the atmospheric pressure because of the integral operation. Moreover, if a surge tank is provided in the pressure supply passage for supressing the pulsation of the pressure, the residual vacuum in the surge tank will have significant affect on the length of time of the closure of the air control valve.
As an example, when the throttle valve is closed so that the engine operation is shifted from an ordinary operating condition to a decelerating condition, i.e., when an operating condition in which the air-fuel ratio is to be controlled to the rich side is started, the air control valve may remain open a long time. As a result, the secondary air is supplied to the engine to produce an over lean mixture which may result in an engine stall.
On the other hand, when the engine operation is shifted from the decelerating condition to the accelerating condition, i.e., when the feedback control of air-fuel ratio is started, the opening of the air control valve is delayed by the effect of the residual atomspheric pressure in the surge tank, and an excessively long time will be required before the air-fuel ratio is controlled to the stoichiometric value.
Thus, if a surge tank is provided in the pressure passsage, it is difficult to avoid such disadvantages in the air-fuel ratio feedback control operation.